Up to now a method for isolating respective elements from each other by means of an insulator has been known as a element isolating method used in a monolithic semiconductor integrated circuit.
In the Japanese patent Application Laid Open No. 59852-1986, for example, a semiconductor device manufacturing method of isolating elements from each other by forming an isolation trench on a bonded SOI (silicon on insulator) substrate is disclosed. This method is the one which makes an SOI substrate by bonding a silicon substrate 42 with another silicon substrate 40 on which surface an insulating film 41 is formed as shown in FIG. 36(A) through an insulating film 41 as shown in FIG. 36(B), forms isolation trenches 43 reaching to the insulating film 41 in the SOI substrate from one main surface of this SOI substrate, then forms an insulating film 44 on the surface of the SOI substrate including inside walls of the isolation trench 43 by thermal oxidation or the like as shown in FIG. 36(C), fills the isolation trenches 43 with polysilicon 45 as shown in FIG. 36(D), removes extra parts of the insulating film 44 and polysilicon 45 which comes out from the isolation trenches 43, and electrically completely isolates respective element areas 46 from the substrate 40 or from each other by means of an insulator as shown FIG. 36(E).
As disclosed on page 66 of a reference paper "Ultra-Fast Silicon Bipolar Technology", for example, a method for forming an isolation trench for isolating respective elements from each other after forming a field oxide film which has a thick portion and a thin portion on the main surface of a silicon substrate is known as a method for forming an isolation trench. The method will be described in the following.
As shown in FIG. 37, the method forms one by one in order a partially trick field oxide film 32, a silicon nitride film 33 and a silicon oxide film to be used as a mask on the main surface of a silicon substrate 31, forms an opening by selectively etching the field oxide film 32, the silicon nitride film 33 and the silicon oxide film 34 in an area where the field oxide film is thin in thickness, then forms an isolation trench 35 by etching the silicon substrate 31 through the opening. And it etches off the silicon oxide film 34 used as a mask, forms an insulating film 36 on an inside wall of the isolation trench 35, then fills polysilicon 37 in the isolation trench 35. Further, it performs etching back of the polysilicon 37 deposited on the silicon nitride film 33 when filling the isolation trench with the polysilicon 37, etches off the silicon nitride film 33, and then forms a silicon oxide film 38 on the top of the polysilicon 37 in the isolation trench (see FIG. 38). In this manner, the method electrically divides completely the silicon substrate 31 into respective element areas by the isolation trench 35 and the insulating film 36.
In a series of manufacturing treatments shown in FIG. 36(A) to 36(E), up to now as described above, an oxide film is generally formed on the surface of an SOI substrate as a mask for forming an isolation trench on the SOI substrate. And the oxide film used as the mask for forming an isolation trench has been removed at once after forming the isolation trench. However as shown in FIG. 36(E), in case of isolating an element area 46 by insulating film 41 from a substrate 40, an insulating film for isolating from the substrate is exposed inside the isolation trench immediately after forming the isolation trench. In this case, since the insulating film 41 inside the SOI substrate for isolating substrate and the oxide film for the mask have about equal etching rate, so that, if attempting to etch off the oxide film as the mask immediately after forming the isolation trench, the insulating film inside the substrate also is etched off at the same time.
The present invention has been made by taking consideration of the above-mentioned actual facts. The first object of the invention is to provide a method for manufacturing a semiconductor device which makes it possible to prevent a local deterioration of dielectric strength in an isolation trench area by protecting the insulating film for isolating between the substrates which is exposed inside the isolation trenches from being etched in the course of the treatment.
The reason why an isolation trench is formed in an area where a field oxide film is thin in thickness as shown in FIG. 37 is that deterioration of flatness of the surface of a substrate is prevented which is feared in case of forming the isolation trench in an area where the field oxide film is thick in thickness. In case of forming an isolation trench in an area where a field oxide film is thick, an end face of the field oxide film is largely exposed by the isolation trench, the field oxide film also is etched when etching off a silicon oxide film used as a mask, and a deep hollow is caused to deteriorate flatness of the substrate. Deterioration of flatness of the substrate causes such a problem as breaking and short-circuiting of a polysilicon wiring or aluminum wiring. Therefore, if an isolation trench is formed in an area where a field oxide film is thin in thickness, deterioration of flatness of the substrate does not cause much of a problem even if the field oxide film is etched.
On the other hand, in case of forming an isolation trench in an area where a field oxide film is thin after forming the field oxide film as in a former method shown in FIGS. 37 and 38, it is necessary to make a semiconductor device for example a transistor to be manufactured larger in size by allowing for some divergence in mask matching so as to surely prevent trouble due to exposing the end face of a thick part of the field oxide film by an isolation trench.
The second object of the invention is to prevent a semiconductor device from being made unnecessarily large in size without deteriorating flatness of the surface of a substrate even if forming an isolation trench in an area where a field oxide film is thick by taking consideration of the above-mentioned problems.
An isolation trench as described above is generally formed by etching a silicon substrate through the R.I.E (reactive ion etching) process. The R.I.E process is a process in which a cathode fall voltage (bias voltage) is generated by applying a high frequency voltage to an electrode on which a silicon substrate is mounted and the silicon substrate is etched by having ions or radicals as active particles generated by being plasma discharged material gas of a fluoride series collide and react against the substrate. Since the R.I.E process being accompanied by a physical etching action is greatly aggressive to the silicon substrate, it caues crystal defects on the surface of inside walls of an isolation trench and the surface of the silicon substrate surrounding the inside walls cause leakage of electric current due to the etching damage. For the purpose of eliminating such crystal defects, up to now an inside wall of an isolation trench has been processed by means of a sacrifice oxidation process or C,D,E (chemical dry etching) process, as disclosed in Japanese patent Applications Laid Open No. 127850-1991 and No. 129854-1991, for example. The sacrifice oxidation process is a process of eliminating crystal defects by etching off an oxide film after forming the oxide film reaching to a depth where crystal defects exist in inside walls of an isolation trench. And the C.D.E process is a process of etching off a part having crystal defects by only a less aggresive chemical etching by means of radicals of material gas activated by plasma discharge.